1. Field of the Invention
The invention relates to speech processing, and more particularly to amplitude adjustment of speech signals.
2. Description of the Related Art
A speech processing signal amplifies a speech signal with a power amplifier to obtain an amplified speech signal with suitable amplitude for speaker broadcasts. However, when the speech signal amplitude is greater than a threshold level, the power amplifier, amplifies the speech signal with a reduced gain, which is referred to as ‘saturation of the power amplifier’. The speech processing signal therefore requires a dynamic range control module to adjust the amplitude of a speech signal before the speech signal is amplified by a power amplifier to prevent the power amplifier from saturation.
A conventional dynamic range control module continuously monitors speech signal amplitude. When the speech signal amplitude is greater than a threshold level, the conventional dynamic range control module attenuates the speech signal before the speech signal is amplified by a power amplifier. The power amplifier is therefore prevented from saturation. The conventional dynamic range control module, however, starts to attenuate the speech signal after the section of the speech signal having amplitude exceeding the threshold level is found. The speech signal section with the high amplitude is therefore still amplified by the power amplifier to obtain an amplified speech signal with a high amplitude, causing amplitude differential between the speech signal section and a subsequent attenuated section. The amplitude difference caused by the conventional dynamic range control module induces a harsh noise in the amplified speech signal.
In addition, a speech signal comprises a series of syllables. Because a conventional dynamic range control module attenuates the speech signal with different attenuation factors according to the speech signal amplitude, when a syllable of the speech signal has different amplitudes, different sections of the syllable are attenuated with different attenuation factors, causing signal distortion in the adjusted speech signal output by the conventional dynamic range control module. Thus, the conventional dynamic range control module has deficiencies, and a new dynamic range control module without the aforementioned deficiencies is required.